Service composition refers to a technology where end user services to be provided to user terminals, e.g. to personal computers, laptop computers, or mobile phones of a telecommunications network, are dynamically built by combining constituent services. The selection of the constituent services as components of the composite service may be performed just in time at user request. The needed constituent services are described in terms of required generic properties. Any service that provides the needed properties can be used as part of the composite service. Rather than fixed binding of a particular service any suitable service within a pool of available services can be selected. The pool of available constituent services can change dynamically by adding new services or by removing them. A composite service can therefore consist of different constituent services at each invocation. Thus, the set of component services that are actually included into a composite service may not be static, but depending on runtime conditions. The constituent services do not need to be specifically designed for service composition. They can be integrated into a composite service, but they can also work as a single service.
From the end user point of view, a telecommunication network is substantially defined by the services it provides. The end user selects desired services from a portfolio of available services. For billing of services provided to the end user, a charging system is provided that is informed by the nodes providing services and applications about user activities. The charging system determines the amount to be charged for the service usage and deducts from the user's account (online charging) or it logs the activity in detail records for later billing (offline charging). Important information used by the charging system is the identification of the user and the details of the provided service, so-called usage information. Various protocols exist to transfer this information from the service nodes to the charging system. Examples are for online charging protocols are RADIUS, standardized for example by the Internet Engineering Task Force (IETF) in the Request For Comments (RFC) 2866, or its successor DIAMETER, standardized by IETF RFC 3588 and in the 3rd Generation Partnership Project (3GPP) Technical Specification (TS) 32.299. The roles in charging of the charging system and service nodes are well standardized for example in 3GPP TS 32.240.
In existing telecommunications networks, the charging system independently evaluates usage information of each service provided to the end user terminal. Such approach of separate service processing is reasonable as long as the service tariff does not depend on the other services provided to the user or on the context of a service usage.
In a service environment that is based on service composition, a service request that starts a composite service triggers the invocation of a number of constituent services, each of which being reported to and handled separately by the charging system. However, it may be desirable that the charging of a composite service involves a more flexible charging than the separate charging of the constituent services.